Differential amplifiers are a popular form of amplifier and are especially useful in analog circuits where they are commonly used as a current mirror and the input stage of operational amplifiers.
A problem with the basic prior art form of differential amplifier is that it tends to have a relatively narrow common-mode operating range that is limited to a fraction of the voltage difference between the V.sub.DD and the V.sub.SS power supply rails of the amplifier circuit.
This limit arises because this form of differential amplifier will not operate properly with a common-mode voltage V.sub.CM of magnitude less than that sufficient to sustain the sum of the gate-to-source voltage V.sub.GS of the input transistors plus the drain-to-source saturation voltage V.sub.DS of the tail current source. In particular, this differential amplifier will not function if EQU V.sub.CM &gt;V.sub.DD -V.sub.DS MIN -V.sub.GS
where V.sub.DS MIN is the minimum allowable drain-to-source voltage of the tail current source necessary to keep the amplifier in saturation. The aim generally with respect to a differential amplifier is to permit satisfactory operation up to a value of V.sub.CM close to that of V.sub.DD, thereby to maximize the operating range, and thus the dynamic range, of the amplifier.
The usual approach to a solution of this problem is to add a second input pair stage, in parallel with the first input pair stage, the second stage being of transistors of the type complementary to those in the first stage, e.g., the first input stage using transistors of p-type and the second input stage using transistors of n-type. Each stage covers a portion of the input common-mode range with the PMOS input stage working down to the lower voltage supply rail V.sub.SS and the NMOS input stage working up to the higher voltage supply rail V.sub.DD. Various schemes have been devised for transforming bias current from one stage to the other stage to minimize the change in the composite transconductance of the two stages. A widely varying composite transconductance is generally undesirable because it creates difficulty in achieving the desired compensation and design results of the complete amplifier. However, such approaches, for success, depend heavily on well-controlled relationships between the characteristics of the NMOS and PMOS devices, which are difficult to realize in practice without a large increase in the manufacturing costs. The present invention involves a different approach.